1. Field of the Invention
The present invention is directed to thermoplastic sheets containing rubber modified styrenic copolymers and the use of said sheets in making thermoplastic items.
2. Description of the Prior Art
Plastic sheet laminates are commonly used in a variety of applications, including identification cards, credit cards, bank cards, key cards, gift cards and the like. Such laminates are generally comprised of a relatively thick core layer containing printed indicia on at least one side thereof, and a relatively thin, transparent layer laminated to each side of the core layer. For certain applications, particularly for credit cards, the laminate is embossed and the raised areas are typically printed or tipped with ink.
These laminates are typically made of plastic sheets containing various thermoplastic blends including primarily polyvinyl chloride homopolymers or copolymers. Typical credit card laminates require properties such as stiffness, impact strength, resilience and flexural durability sufficient to provide about a one- to two-year service life.
In some applications, the printing and/or printed indicia is applied on the protective or laminated layer and then reverse-laminated onto the core layer.
As an example, U.S. Pat. No. 5,543,466 to Norman discloses laminates for use in credit card applications that have at least one core layer and at least one over layer on each side. The core layer composition includes polyvinyl chloride, a copolymer derived from vinyl chloride and vinyl ester monomers, at least one acrylic resin polymer, as well as a copolymer derived from ethylene and vinyl acetate monomers. The core layer is corona treated in order to improve ink adhesion thereto. The overlay composition is similar but generally is transparent. The laminated credit card can optionally contain a magnetic strip on the over layer or a microchip therein.
A particular drawback to polyvinyl chloride based cards is their generally poor printability or ink acceptability. As indicated in the Norman patent, the core layer requires corona treatment in order to improve ink adhesion and is further totally laminated to protect the printed layer. The extra processing steps required for protective lamination add considerable cost to the final card. The elimination of these steps and their added cost is an ongoing objective in the art.
U.S. Pat. No. 4,772,667 to Biletch et al. discloses a thermoplastic polymer that includes a styrenic monomer, an acrylate, a methacrylate and a block copolymer.
U.S. Pat. No. 5,290,862 to Blasius discloses a polymer alloy that contains from 30 to 83 weight percent of a brittle polymer; from 3 to 50 weight percent of a rubbery polymer; and from 15 to 67 weight percent of a ductile polymer, provided that the ductile polymer and the rubbery polymer are at least compatible.
U.S. Pat. No. 5,891,962 to Otsuzuki et al. discloses a transparent, rubber-modified styrene resin that contains 70 to 96 parts by weight of a copolymer formed of 20 to 70 wt. % of styrene monomer units and 30 to 80 wt. % of alkyl (meth)acrylate monomer units and 4 to 30 parts by weight of a rubbery polymer. The rubbery polymer is dispersed in the copolymer as particles and the copolymer and the rubbery polymer have substantially the same refractive index.
Heretofore, the above described thermoplastic polymer compositions have not been used in sheet and/or disposable card applications because they do not provide the required stiffness, impact strength, resilience and flexural durability properties.
It would be desirable to provide a low cost thermoplastic sheet material that can be used in disposable card applications that provides sufficient stiffness, resilience and flexural durability properties, while also having ink printability and ink acceptability properties such that a lamination layer is not required to protect the printed image.